STJs are used for detecting particles such as photons, ions, molecules, biological fragments or other energetic particles, and have a fast response and high detection efficiency. Examples of known STJ detectors are described in “Quasiparticle trapping in a superconductive detector system exhibiting high energy and position resolution”, Kraus et al, Physics Letters B, Vol. 231, No. 1,2, November 1989, pages 195–202; and “Superconducting particle detectors” Booth and Goldie, Supercond. Sci. Technol. 9 (1996) 493–516.
In order to understand the problems addressed by the present invention, the principles of operation of a STJ will now be described.
This discussion applies to the measurement of mono-energetic quanta that interact in a superconductor and are read out using a superconducting tunnel junction (STJ).
To detect incident energy from photons or other particles, a small magnetic field is applied in the plane of the tunnel barrier to suppress the zero voltage supercurrent. The STJ is electrically biassed (voltage bias is preferred) at a voltage less than the sum of the superconducting energy gaps of the films comprising the STJ. For a symmetric STJ, this meanseVb<2Δ                where e is the electronic charge, Vb is the bias voltage and Δ is the superconducting energy gap.        
When energy is deposited in one of the superconductors, Cooper pairs are broken leading to an increase in the number of quasiparticle excitations. These excitations are detected as a temporary increase in the tunneling current. The magnitude of the tunneling current that produces the detected signal is determined by the excess number of excitations, proportional to the deposited energy, and the quasiparticle tunneling rate. It follows from the details of the solution of the tunneling Hamiltonian including the initial and final densities of states and the distribution of quasiparticles as a function of energy, that the optimal bias point for maximum tunneling rate (and hence maximum signal) occurs at about       eV    b    ≈            Δ      6        .  For real STJs, interaction of the ac Josephson super-current occurs with standing electromagnetic waves that are sustained in the cavity formed by the tunneling barrier. This interaction produces sharply peaked, zero frequency, currents in the STJ I–V that are known as Fiske resonances. For a two dimensional cavity of side length L, width W, the current peaks occur at voltages given by       V    Fiske    =            h              2        ⁢        e              ⁢                  c        _                    2        ⁢        L              ⁢                            n          2                +                                            m              2                        ⁡                          (                              L                w                            )                                2                    Here, {overscore (c)}≈0.03c is the electromagnetic wave velocity in the barrier, and h is Planck's constant. The resonant peaks lead to regions of negative resistance in the STJ I–V that can produce bias instability. The odd order resonances (m+n odd) are particularly troublesome since they are out of phase with the periodic modulation of the zero voltage supercurrent by the applied field. The STJ is unconditionally stable if the device is operated in a magnetic field to fully suppress the zero voltage supercurrent and at a bias less than the first [1,0] resonance.
This means that the maximum device size consistent with stable operation and maximum signal is given by   L  <                    6        ⁢        h        ⁢                                  ⁢                  c          _                            4        ⁢        Δ              .  For a Ta-based STJ with Δ=600 μeV, we find L<90 μm.
Technologically useful devices need to have maximum area. A solution to this has been previously suggested for a simple STJ where the detector is sub-divided into smaller sub-units (“Fiske modes in superconducting tunnel junction detectors”, Friedrich et al, Nuclear Instruments and Methods in Physics Research A 444 (2000)151–155). These are simultaneously biased in parallel, and the first resonance occurs at a voltage characteristic of the sub-unit size. This solution was recognized to be possible only at the expense of overall detector energy resolution. There are inevitable small differences in the responsivity of each of the pixels leading to a broadening of the device response function to mono-energetic inputs, and a reduction in energy resolution.